The depth of the prior art elongated fluidized bed furnace is 1 mm at most in Japan and 2 mm at most in other countries.
With a furnace having an excessive depth, the state of flow varies depending on the depth due to such causes as the pressure difference between top and bottom portions of the fluidized bed of the furnace and joining of bubbles.
(1) When the rate of flow of flowable gas is low
Although there is an apparently satisfactory state of flow of bubbles in a top portion of the fluidized bed, there is no substantial flow in a bottom portion of the bed because of a small volume of bubbles.
(2) When the rate of flow of flowable gas is increased
Flow tends to be produced in a bottom portion of the fluidized bed. In a top portion of the bed, however, joining of bubbles to one another takes place to produce a state of considerably strong flow. With a furnace having a small diameter, there is produced a slug flow state occupied by gas slug over the entirety of a certain furnace height range.
(3) When the rate of flow of flowable gas is further increased
Slug flow tends to be produced even in a bottom portion of the fluidized bed. In a top portion of the bed, a more violent slug flow is produced, and the top surface of the bed is violently moved up and down with the rise of gaseous slug.
A satisfactory state of flow in the fluidized bed is a bubble flow. Where the state of flow is different between the top and bottom portions of the fluidized bed as in the cases (1) to (3) described above, the following problems are presented:
(1) The temperature uniformity of the fluidized bed is deteriorated.
(2) It is difficult to obtain a uniform atmosphere in the bed.
(3) Usually, elongated objects to be processed are inserted into the fluidized bed by a hanging system. When the flow in the top portion of the bed becomes violent, it becomes difficult to hold the object being processed in a vertically fixed position due to a rise of the and bubbles.
(4) When a slug flow is produced, the top surface of the fluidized bed is displaced up and down, and powder used for the fluidized bed, e.g., alumina, is spattered.